1. Technical Field
This invention relates to the field of providing high speed digital data services to a digital service subscriber and, more particularly, to so-called asymmetric digital subscriber line (ADSL) services and to a method of reducing near-end crosstalk in discrete multi-tone (DMT) modems located at a central office and at the subscriber""s premises for providing ADSL services.
2. Description of the Related Arts
In the field of cable television, cable modem technology is emerging which provides increased bandwidth services to the home. Cable television equipment manufacturers are promoting the upgrading of cable television distribution plant to comprise so-called hybrid optical fiber and coaxial cable (hybrid fiber/coax) facilities. It is anticipated in the cable field that bandwidths to and from the subscriber can be increased so that bidirectional voice and data services may be provided in addition to traditional cable television programming. In the related field of telecommunications, there exists a considerable amount of embedded distribution plant comprising high capacity twisted wire pair cable. Historically, each cable pair was specially loaded with inductance coils at periodic intervals along the path from a serving central office to the subscriber""s premises to improve voice telephony performance. The inductance loading countered the effects of the high capacity cable and provided a flat bandwidth on each twisted cable pair at voice frequencies. On the other hand, frequencies higher than voice bandwidth were intentionally attenuated to such a degree that the twisted wire cable pair was unusable for other than a voice channel. When the loading is removed, the twisted cable pair bandwidth improves and becomes more comparable to that of coaxial cable.
An emerging technology in the telecommunications arts that competes with cable modem technology is so-called asymmetric digital subscriber line (ADSL) technology. Referring to FIG. 1 taken from American National Standards Institute standards document T1.413-1995, there is shown a public switched telecommunication network (PSTN) 105 and a digital network (for example, a frame relay, asynchronous transfer mode (ATM), Internet or other digital network) 110 at the left. At the right is the subscriber""s premises. The digital network 110 is coupled via a logical interface V to an ADSL transceiver unit (ATU) at the serving central office (C). Also at a serving central office are located a splitter 120 for splitting the telecommunications services from the digital services, typically based on frequency. For example, a voice channel may still be preserved at from 0-4000 Hz. The splitter function may be integrated into ATU-C 115 (and at the remote subscriber site, into ATU-R 135). Interface U-C represents the subscriber loop (twisted pair) interface at the central office C and interface U-R represents the subscriber loop interface at the remote subscriber terminal end of the twisted wire cable pair or other facility 125. Facility 125 may comprise, for example, a twisted wire pair or a hybrid optical fiber/twisted wire pair facility or other wired or wireless facility having comparable or greater bandwidth. Service module (SM) 150 or 155 at the remote location may comprise an intelligent telecommunications terminal, a personal computer, a television terminal, an energy management system, a security system or other service module known in the art. Plain old telephone service is (POTS) module 145 represents a traditional telecommunications terminal such as a facsimile terminal, voice bandwidth modem or standard telephone. Facility Cl distribution 140 within the subscriber premises may comprise, for example, a bus such as a home bus or a star network or other configuration. By bus as used herein is intended a communications link that may be wired or wireless connecting a plurality of devices together. The bus may be arranged so that there is contention for access to the bus according to priorities or be provided sufficient capacity to alleviate the likelihood of contention. Interface T represents the interface between a service module (SM) and/or a bus/star 140 to other service modules (SM""s).
Referring to FIGS. 2A and 2B, there are shown respectively an ATU-C transmitter whose reference diagram is taken from A.N.S.I. T1.413-1995 and an ATU-C receiver whose reference diagram is derived therefrom. In FIG. 2A, there is shown an expanded functional block diagram of the transmitter portion of ATU-C 115 of FIG. 1. A multiplexer/sync control unit 200 provides the interface to the digital network 110. Various high speed data rate links AS0, AS1, AS2 and AS3 at multiples of 1.536 Mbits/sec are provided toward digital network 110. In particular, each AS link represents an independent downstream simplex (unidirectional downstream) bearer of data traffic. Lower speed data services are also shown and represented by LSO (16 or 64 kbits/sec), LS1 (160 kbits/sec) and LS2 (384 or 576 kbits/sec). Each LS link may represent a duplex bearer (bidirectional) carrying both downstream and upstream traffic or, in the alternative, a unidirectional simplex bearer.
CRC 205 and CRC 210 represent cyclic redundancy check in each direction of transmission. Scrambler and forward error correction 215, 220 represent scrambling and forward error correction, for example, using Reed-Solomon error correction coding, in each direction of transmission. Interleaver 225 provides a data interleaving function as is further described in A.N.S.I. T1.413-1995, incorporated by reference as necessary. Tone ordering function 230 provides tone selection and control functions as are also described by A.N.S.I. T1.413-1995. Constellation encoder (if used) and gain scaling functions are represented by block 235. The inverse discrete Fourier transform function applied for data modulation is represented by block 240. Two data directions are shown coupling IDFT 240 and output parallel to serial buffer 245 where a cyclic prefix is added to each data frame. Finally, a digital to analog converter and analog signal processing function are represented by block 250 which interfaces the subscriber facility 125.
Referring to FIG. 2B, the ADSL receiver at the central office is shown. The horizontal arrows are reversed in direction from FIG. 2A. Data demultiplexer 255 interfaces the digital network 110. Descrambler 265, 258, deinterleaver 270, decoder 280, DFT 285, input serial to parallel buffer 290 and analog to digital converter 295 represent the significant changes in function between ATU-C transmitter and receiver.
Referring to FIG. 3, at a subscriber terminal, the transmitter (ATU-R) is similarly configured as ATU-C but it is assumed that channels operate at LS0, LS1 or LS2 toward the subscriber""s equipment. Cyclic redundancy checks 305/310 are provided for each direction of transmission to/from subscriber equipment 375. Scrambler and forward error correction circuits 315 and 320, for example, using Reed-Solomon error correction coding, are provided for especially secure data transmission. An interleaver 325 is provided in one transmit path. Tone ordering circuitry 330 is necessary for generating and ordering the discrete multi tones of the discret multi-tone (DMT) modem. The constellation encoder and gain scaler 335 may or may not provide a form of trellis data encoding and gain scaling for controlling the tone ordering. IDFT block 340 performs an inverse discrete Fourier transform for modulating the digital data. The output parallel to serial buffer 345 is provided for providing parallel to serial conversion to a digital to analog converter and analog signal processing interface 250 which interfaces the subscriber loop 125.
The analog signal framing (FIG. 4) used in ADSL technology is obtained by passing quadrature amplitude modulation (QAM) samples through a D/A converter 250 or 350. These samples are arranged in a superframe of 69 frames (frames 0-68) totaling approximately 17 milliseconds. Altogether 512 samples (256 real and 256 imaginary, 0-511) are taken of the data. An additional 32 samples contain a cyclic prefix, making a total of 544 samples in each frame. The cyclic prefix CP 401 is added, for example, to signal 402 of frame 0 (FIG. 4) at the output parallel/serial buffer 245 and 345 shown in FIGS. 2A and 3 respectively.
Every 69th frame contains a pseudo-random number (PRN) sequence with a nominal length of 544 samples. This PRN sequence (the so-called synch symbol) permits recovery of the frame boundary after interruptions.
The sub-carrier tones are spaced at 4.3125 kHz according to the ANSI Standard T1.413-1995 and at carrier 64 where the frequency is 276 kHz, a pilot carrier is inserted. The data modulated on that pilot is a constant bit value (for example, 0,0). Other details of frame construction, data modulation, tone ordering and the like may be found in the Standard and are not believed to be particularly relevant to the principles of the present invention.
Near-end crosstalk, hereinafter referred to as NEXT, is a potentially severe problem for operating multiple Digital Subscriber Line (DSL) modems over twisted-pair wires between a Central Office (CO) and a subscriber""s location. NEXT occurs when the transmissions from one or more modems, particularly those at the central office, capacitively couple into each other""s twisted-wire pairs and impair the ability of those modems to receive transmissions from the other end of the tvisted-wire pairs. Moreover, with severe NEXT, a subscriber modem cannot receive transmissions from its transmitting central office modem. This problem is most severe when a modem operates in full-duplex mode in which transmission is simultaneously bi-directional at all frequencies.
Cables that serve subscribers and terminate at a central office can comprise thousands of twisted wire pairs that are bundled together in a limited cross-sectional (typically circular) area. Electrical signals traveling on the twisted pairs can easily electrically couple into physically proximate twisted pairs, consequently, near end crosstalk has a detrimental effect on bit error rate. Since the concept behind ADSL technology is to optimize bandwidth use, the phenomenon of near end cross-talk limits any one of three factors: the distance a subscriber can be from the central office, the digital data rate of service and the bit error rate of any digital data service. As the number of subscribers increase to ADSL technology, the likelihood will increase that an ADSL subscriber will be served by a twisted wire cable pair proximate to that of another subscriber and that frequencies from one twisted wire cable pair will adversely impact the signal to noise ratio of digital signals on an adjacent or proximate cable pair. Consequently, there is a need in the art to alleviate the effects of NEXT.
One way to reduce NEXT might be to assign cable pairs to subscribers in such a way that subscribers to ADSL services are not in the same bundles as other subscribers. Of course, at some point in time, as subscribers to ADSL increase, so does the likelihood that cable pair assignment in such a manner cannot be accomplished. Thus, there is a need in the art for reducing NEXT in ADSL services.
The present invention offers the possibility of substantially improving one or more of the following parameters: distance of the subscriber from the central office, data rate, and error rate. For example, the distance of the subscriber from the central office may be increased while the data rate and error rate are held constant. According to the principles of the present invention, the framing suggested by ANSI T1.413-1995 is modified and frame alignment is recommended for all frames transmitted by the central office. Also, all frames transmitted by subscriber modems should be aligned to coincide with received frames. An echo canceler may be applied to cancel near end echo as will be further discussed herein. Interspersing transmit frequencies (individual frequencies or bands) between central office and subscriber modems may further reduce the effects of near end crosstalk.
In particular, in accordance with the present invention, the length of the cyclic prefix prepended to each frame of a DMT frame should be increased to a value that is at least the length of the sum of 1) the maximum round trip delay from a central office to a subscriber and 2) the delay required to prevent intersymbol interference or ISI (according to the ANSI standard, 32 samples or 14.5 microseconds to prevent ISI). For example, if the sampling rate is maintained at approximately 2.2 megaHertz and if the maximum round-trip delay is approximately 80 microseconds for an 18,000 foot twisted-wire pair loop, one might increase the cyclic prefix to about 184 samples from the recommended 32 samples or the equivalent of 81.3 microseconds of signal duration or even higher depending on concerns about ISI.
Also, it is advantageous to increase the length of the frame proportionately to the increase in the cyclic prefix. For example, if the cyclic prefix is lengthened from 14.5 microseconds to 81.3 microseconds, then the frame should be lengthened from 250 microseconds to approximately 1 millisecond. If the same sampling rate is maintained (at around 2.2 megaHertz), then other parameters will change accordingly. If the frame length is increased to 1 millisecond, the number of sample points of the IDFT should increase from 512 to, for example, 2048, the carrier spacing should decrease, for example, from 4.1325 kHz to 1.089998 kHz and the DMT""s sampling rate should increase slightly from 2208 kHz to 2263 kHz. These numbers are representative only, and other practitioners of the art may deviate depending on the application.
It is a further principle of the present invention to align frames transmitted from the central office toward the subscriber. All frames transmitted by all central office modems are aligned and synchronized to begin at the same time as they are transmitted toward subscribers. That is, all frames transmitted by all central office modems to all subscribers (especially served on the same cable of twisted wire pairs) start and end at the same times. One way to accomplish the alignment is to provide a common shared timing signal to all CO modems from a common source. For example, the signal may comprise a train of narrow, for example, 1 microsecond pulses spaced by the period of a frame (for example, one millisecond). Each central office modem then would align its frames that are transmitted toward respective subscribers over different twisted wire cable pairs with this pulse train.
The pulse train from the common source may be accompanied by a higher frequency signal (for example, a sinusoid or a pulse train at the frequency specified in Section 6.9 of ANSI T1.413-1995) to which each modem could be synchronized. Alternatively, the pilot tone of 276 kHz specified at Section 6.9.1.2 might be applied as the shared resource. Either alternative will save the costs of providing an oscillator within each central office modem.
Now, it is a further principle of the present invention that all frames transmitted by the subscriber""s modem toward the central office modem be aligned with the frames that it receives from the central office. Its transmitted frames should coincide with frames it receives from the central office as may be seen from the FIGS. 6a and 6c. 
With the frames aligned as described and per FIG. 6 and with the lengthened cyclic prefix (for example, 184 samples long), each central office modem samples its received frame wholly within the frame 1) received from its subscriber""s modem and 2) transmitted by all other subscriber""s modems as in FIG. 5 which shows that each central office modem samples its received frame wholly within the frames transmitted by all other subscribers"" modems.
Referring again to FIG. 5, there is shown the frames that a central office modem would receive if the principles of the present invention were followed for two extremes of round-trip delay. When the subscriber is almost adjacent to the central office, the leading edge of the subscriber frame is received 184 samples before the leading edge of the central office sampling. The leading edge of the distant subscriber""s frame coincides with the leading edge of the CO""s sampling. For these and any delays between these extremes, the CO modem receiver takes its samples entirely within both the frame received from all subscribers"" modems and the frames transmitted by all modems at the central office. Thus, a frame transmitted at one carrier frequency is guaranteed to not interfere with a frame received at another carrier frequency. Taking samples from inside the frame preserves the orthogonality of the sinusoids in the discrete Fourier transform (DFT), since the modem""s DFT modulator operates on an integral number of periods of each carrier frequency. NEXT is reduced if not eliminated via this orthogonality requirement.
Finally, it is a further principle of the present invention that the central office and subscriber modems"" transmit frequencies may be interspersed. For example, one or more bands of frequencies toward the subscriber from the central office may be interspersed with a band of frequencies in the return direction from the subscriber modem to the central office. At the extreme, all odd multipled carriers may be used in the downstream path from the central office and even multipled carriers be used in the upstream path (or vice versa). That is, for example, odd multiples of 1.089998 kHz may be used in the downstream and even multiples in the upstream path exclusive of any band reserved for POTS telephony. This even-odd distribution of upstream and downstream frequencies offers the advantage of degrading nearly symmetrically as distance from the central office and/or NEXT interference increase.
Referring again to FIG. 6, it may be seen that similar benefits are obtained at a subscriber modem. A subscriber""s modem will sample only inside the frames transmitted by itself and by other subscriber modems of the same type. Again, sampling inside the frames preserves the orthogonality of sinusoids at adjacent frequencies.
Further advantages and features of the present invention will be understood from studying the accompanying drawings and from reading the following detailed description thereof